The present invention relates to a damper. More particularly this invention concerns a damper used on the actuating rod of a vacuum switch of a tap changer.
Vacuum switches, also known as vacuum switching cells, are used in high-current applications as they avoid the arcing and burning problems of contacts that are switched, even if in a bath of dielectric oil. Such switches are particularly useful in tap changers which make and break connections while under considerable load.
A force-storage unit is typically provided to operate an actuating rod of such a vacuum cell rapidly at an exactly predetermined time in the switching cycle. The actuating rod must be moved perfectly accurately to avoid stress to the bellows connecting it to the envelope of the vacuum switch. Furthermore the actuating movement must, on opening of the vacuum switch, start brusquely to separate possible welded contacts, but must close gently to avoid overtravel and damage to the switch. In order to achieve such carefully controlled movement, it is standard as described in U.S. Pat. No. 3,553,395 of White to provide a dual dashpot damper that is connected to the vacuum-switch actuating rod. Such a damper comprises two pistons mounted on the axially extending actuating rod and reciprocal axially in respective compartments. The cylinder is formed in each compartment with a small radial port through which the fluid, typically the oil in which the entire system is immersed, is pumped as the piston moves back and forth. The restriction to flow constituted by each of the ports damps the motion of the pistons. When the ports are of different sizes, the damping action for one direction is different from that for the opposite direction.
Commonly owned U.S. Pat. No. 5,107,200 of Dohnal describes a switching system for a step transformer having at least two adjacent taps and a pair of terminals shiftable between the taps. The switching system or tap changer has a pair of fixed contacts normally connected to the terminals, a vacuum switch or interrupter connected between the terminals and displaceable between an open-circuit position and a closed-circuit position, a pair of movable contacts each engageable with a respective one of the fixed contacts and forming therewith a respective bypass switch, and a cam rotatable about a cam axis and having a contact face and an axially oppositely facing interrupter face each formed with a respective operating formation. A drive rotates the cam about its axis through steps of a half revolution. A respective cam follower engaged between each of the movable contacts and the contact-face formation opens and closes one of the bypass switches on rotation of the cam through a half revolution and thereafter opens and closes the other of the bypass switches on rotation of the cam through a succeeding half revolution. Another cam follower and a force-storage unit engaged between the interrupter and the interrupter-face formation snap the interrupter open and then snap it closed each time the cam is rotated through a half revolution. This system has a damper for movement of the actuating rod at the ends of its stroke. This damper includes a cylinder fixed on the housing and a piston carried on the rod and subdividing the cylinder into a pair of generally closed compartments. Each of the compartments is formed with a vent aperture of limited flow cross section.
Such dampers have certain disadvantages. Principally their reaction characteristicsxe2x80x94movement speed, return rate, resistance forcexe2x80x94vary considerably with temperature. This is mainly caused by the decrease in viscosity of the oil they use as the temperature of this oil increases. In the known switching environment, there can be considerable heat build up so that, for instance, a system that has sat for quite some time, so that it is basically at ambient temperature, heats considerably during a period of frequent tap changing. Thus to start with it will work sluggishly and cause the contacts to open and/or close late, whereas after some time there will be virtually no damping action.
Another problem with these systems is that it is very difficult to alter their damping characteristics. Thus if the application is changed, the entire damper must be switched out against a new one. Furthermore the known dampers are fairly bulky, especially in the axial dimension, so that accommodating them in the tight surroundings of a tap changer is fairly difficult.
It is therefore an object of the present invention to provide an improved damper for a tap-changer vacuum switch.
Another object is the provision of such an improved damper for a tap-changer vacuum switch which overcomes the above-given disadvantages, that is whose damping characteristic does not change significantly with temperature, that can be reset easily, and that is of simple, inexpensive, and compact construction.
A tap-changer vacuum switch having an actuating rod extending and displaceable along an axis is provided according to the invention with a damper having a damper housing offset from and fixed relative to the vacuum switch, and a rod piston fixed on the valve-actuating rod, in the damper housing, and defining in the damper housing a compartment whose volume changes as the rod piston moves axially with the rod. The damper housing is formed with a pair of radially open ports opening into the compartment. An in-only check valve fitted to one of the ports only permits fluid flow into the compartment, and an out-only check valve fitted to the other port only permits fluid flow out of the compartment.
The damper housing in accordance with the invention is formed at each port with an outwardly open seat. The check valves are retained by releasable means such as bolts in the respective seats. Thus they can easily be switched out for valves with different opening and closing characteristics.
The seats according to the invention are circular and identical. The check valves have identical end flanges fitting complementarily in the seats. Furthermore the valves can have opposite ends each formed with such a flange so that they can both be of identical construction, one simply mounted oppositely to the other. This further reduces the complexity of the system since for a given opening or closing pressure only a single valve need be stocked.
The valves can be held in place by collars or shoulders on respective retaining bolts that engage the respective end flanges and press same into the respective seats. Seal rings under the flanges prevent any unwanted leakage.
Each of the valves according to the invention includes a tubular valve housing formed with the respective flange and having one end formed with an inwardly directed valve seat, a cup-shaped valve piston displaceable in the valve housing into and out of engagement with the respective valve seat, and a spring in the valve housing urging the respective piston toward the respective seat.
Thus with this system the opening and closing pressures for these valves are largely determined by the constants of their springs. Since the spring constant is much less susceptible to change as its temperature changes, this means that the valves will perform uniformly whether hot or cold. Furthermore the check valves are mounted on the side of the damper housing, in a location where there is normally ample room, so that they do not make the overall assembly longer.